This invention relates to feedback cancelling circuits and, in particular, to a circuit for reducing acoustic feedback in public address systems and in hearing aids.
A public address system is an "open loop" system in which sound is converted by a microphone into an electrical signal which is amplified and converted back into sound waves by one or more speakers. Sound waves are slight variations in air pressure which the microphone converts into an electrical signal of varying amplitude.
In theory, a signal passes through a public address system once, never to return. Outdoors and in well designed auditoriums or concert halls, this is essentially true. In other situations, a significant level of sound reaches the microphone from the speakers. When the output of an amplifier is coupled to the input of the amplifier, one has feedback, a closed loop with the potential to oscillate.
Acoustic feedback in a public address system can cause a mild echo or a self-sustaining ringing, depending upon the loudness of the sound returning to the microphone. The cause of the feedback can be poor placement of a speaker relative to the microphone, walls that reflect sound, and/or simply having the volume set too high on the amplifier.
In a hearing aid, a microphone is connected to a speaker by a high gain (60-80 db) amplifier and is quite close to the speaker in a fitted earpiece. The earpiece is assumed to fit the ear canal exactly and the tissue of the ear canal is relied upon to isolate the speaker from the microphone. If the earpiece should move slightly and not seal the ear canal, an acoustic path is opened, connecting the speaker to the microphone. The misalignment of the hearing aid manifests itself as an unpleasant squeal that is audible even to those several feet from the wearer. The squeal is eliminated by reducing the gain of the amplifier by way of an external volume control on the hearing aid. Often the wearer is obliged to adjust the gain frequently as the loudness of background sounds and sounds of interest changes. While feedback is an annoyance in a public address system, feedback in a hearing aid can be more serious since it interferes with hearing and may cause the wearer not to use the hearing aid. High level feedback in a hearing aid may even damage the already impaired hearing of the wearer.
There are two difficulties to eliminating feedback in an acoustic system. One difficulty is determining whether the sound passing through the amplifier is an echo or an original sound and the second difficulty is determining the travel time of the echo. In the prior art, a variety of systems have been proposed for detecting an echo, typically assuming that a single frequency tone of large amplitude is an echo. When an echo is detected, either the gain of the amplifier is reduced or the signal from the microphone is filtered to eliminate the tone. In a hearing aid, reducing the gain temporarily shuts off the hearing aid causing a silent gap in what is heard. Filtering out a frequency or band of frequencies can have the same effect if the frequencies happen to be those which need amplification to be heard. Some systems in the prior art have a calibration mode for determining the time delay of an echo in order to cancel the echo. These systems are not amenable to being incorporated into a hearing aid.
In view of the foregoing, it is therefore an object of the invention to provide a feedback cancelling circuit which does not squelch or reduce the gain of an amplifier.
Another object of the invention is to provide a feedback cancelling circuit which operates independently of the delay of the echo.
A further object of the invention is to provide a feedback cancelling circuit in which an original sound is reconstructed from an inaudible part of an echo and is subtracted from the audible part of the echo, thereby cancelling the echo.